Radio broadcasting is done in frequency band "channels" which, in Europe, are 9 kHz wide and, in North America, are 10 kHz wide. Each broadcast signal has 2 "sidebands" and sometimes the upper sideband of one transmitter's signal overlaps and interferes with the lower sideband of another transmitter's signal, e.g. when the second transmitter is using the adjacent higher channel.
Due to non-linearities, between the antenna and the mixer, in the signal path of signals to be received, "mixed products" arise from signals which are not intended to be received. Those mixed products, the frequency of which lies within the usable frequency range of the respectively selected transmitter, appear interferingly in the form of intermodulation.
The interference effect becomes more severe, the stronger the circuits containing the non-linearities are driven. Accordingly, the interference effect of intermodulation can be reduced by reducing the gain of the input circuit. However, to do this, it is necessary to determine whether intermodulation is present.